Method of treating a workpiece

ABSTRACT

Magnetic chuck for holding a workpiece, the workpiece often being distorted by the great magnetic force necessary for holding it, and releasing the holding force in steps while reducing the force of the working operation thereon.

United States Patent Inventor Appl. No. Filed Patented Assignee METHODOF TREATING A WORKPIECE OTHER REFERENCES Rusinoff, ManufacturingProcesses, 1962, pp. 613, 616- 618.

Primary Examiner-John F2 Campbell Assistant ExaminerRobert J. CraigAttorney-Paul H. Gallagher 4 Claims, 7 Drawing Figs. U.S.Cl 29/558,ABSTRACT: Magnetic chuck for holding a workpiece, the 29/557, 51/326,317/453 workpiece often being distorted by the great magnetic forcelnt.Cl 823 13/04 necessary for holding it, and releasing the holdingforce in Field of Search 29/558; steps while reducing the force of theworking operation 269/8; 51/281, 326, 327; 317/453 thereon.

A B 2 C flan.) 2101 2 8 DE-ENE/i'a/zzn MAG/vs 17250 GREATLY EXAGEIPRAT zD E 210a. 2/01, 509 .3-

M -NER IZED MAGNET/ZED 11 2/042 2 8 fl L g L W i.- T 4 210e PATEN-TEU141112 31911 SHEET 2 OF 3 METHOD or TREATING A worumscs CROSS REFERENCESThis application is a division of my copending application, Ser. No.277,253,frled May 1, 1963.

The invention has particular adaptation to such installations as machinetools, in which a workpiece being worked on is held on a chuck bymagnetism. In order to remove the workpiece from the chuck it is, ofcourse, necessary to demagnetize the chuck. However, in the overalloperation of performing a work operation on a workpiece, it may becomenecessary, in addition to simply demagrretizing the chuck and workpiece,to partially demagnetize, and again remagnetize them, as when it isdesired to remove the workpiece for checking it, or checking theoperation thereon, or for other reasons.

A broad object of the invention is to provide apparatus and methodfor-magnetically holding a workpiece, and relieving the holding force insteps, while also reducing the force of the working operation thereon,whereby the distortion of the workpiece, which is often unavoidable, andthe force of the working operation are simultaneously reduced, resultingin a more accurate working operation on the workpiece.

Another object is to provide apparatus and method of the foregoinggeneral character wherein a workpiece may be demagnetized to a selectedand controlled degree relative to full magnetization.

Still another object of the invention is to provide apparatus and methodof the foregoing general character which includes an arrangement fordemagnetizing in individually controlled steps whereby to enable a workoperation on a workpiece after each such demagnetizing step, to therebyenable accurate finishing of the workpiece in a manner'to overcomepossible distortion thereof due to secure holding of the workpiece onthe chuck by full or greater magnetization.

Another object is to provide magnetizing and demagnetizing apparatus,including cycling means operative selectively for magnetizing a chuck tofull magnetization level or a lesser controllably adjustable level, forproducing steady reverse current to demagnetize the chuck to apredetermined lesser level of magnetization, and for demagnetizing thechuck in a series of steps of successively reverse currents to a levelnear zero, each according to a predetermined law of operation.

Other objects and advantages of the invention will appear from thefollowing detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is an electrical diagram of the circuit utilized in one form ofthe invention;

FIG. 2 is an electrical diagram of the circuit utilized in another formof the invention;

FIG. 3 is a detail view of switching mechanism utilized in thisapparatus;

FIG. 4 is a detail view of switch means illustrated in FIG. 3;

FIG. 5 is a diagrammatic view illustrating motor and cam means;

FIG. 6 is a fragmentary illustration of certain control switchpositions; and

FIG. 7 is a diagrammatic view, greatly exaggerated, showing certainsteps in a working operation on a workpiece. Referring now in detail tothe drawings, attention is directed first to FIG. 1, illustrating theelectrical circuit utilized in one form of invention. In this circuitdiagram, an AC source is indicated at 10, with lines leading to atransformer 12 through a tapped terminal strip 14, for providingselective voltage connections to the primary 12p of ,a transformer 12.Preferably the primary 12p includes two separate coils which may beconnected in circuit selectively or together. The transformerl2 alsoincludes two secondaries 12d and l2s2. Connected across a po tion of thesecondary 12s1 is another transformer. 16, which includes a primary 16pand a secondary 16s.

"The apparatus of FIG. 1 includes portions of the kind of demagnetizingapparatus disclosed and claimed in my prior Pat. No. 2,825,854, datedMar. 4, 1958, and includes the broad concept of an AC source, convertingthe AC to DC and transmitting it to the electromagnet, interrupting theAC and while the AC is interrupted, reversing the DC and at eachreversal reducing the voltage applied to the electromagnet. In theapparatus disclosed in FIG. 1, the main components of the apparatus justreferred to include a motor 18, a rotary tap switch 20, and switchingmechanism 22 driven by the motor; the magnetic chuck is indicated at 214which, as referred to again hereinbelow, is of the permanent magnettype.

Referring to the various components of the circuit of Flg. lindividually, the motor 18 is driven from the secondary 12s2, through arectifier 26 and other elements to be referred to hereinbelow. The motoris associated with and controlled by a plurality of limit switches 27,29, 31 and 33 (see also FIG. 5) actuated by corresponding cams 35 drivenby the motor, in conjunction with the operation of other portions of thecircuit, to be described more fully hereinafter. The connection betweenthe motor 18 and the rotary tap switch 20, indicated diagrammatically at28, may be of any suitable arrangement, such as disclosed in my priorpatent referred to above.

The rotary tap switch 20 includes an arm 3 driven by the motor 18,successively engageable with contact terminals 32 arranged in a circularseries. The arm 30 is arranged for moving off of each contact terminalto a space between adjacent terminals to an OFF position, and thereafteronto the next contact terminal. The contact terminals 32 are connectedwith individual tappings of the secondary l2sl through respectiveconductors 34 for producing different voltage values and applying themto the chuck 24 in a de'magnetizing operation.

Conductors 36 and 38 lead fromthe secondary 12:1 to a rectifier 40 forconverting the AC to DC which thereupon is transmitted to the magnet inthe chuck; the conductor 36 is connected directly with one end of thesecondary'and leads through a fuse 42 to one junction of the rectifier,while the other conductor 38 leads from the arm 30 of the rotary tapswitch 20 through an AC interrupting'switch 44 and through conductor 46to the opposite junction of the rectifier. The switch 44 is incorporatedin the switching means 22 referred to above, which also includesreversing switches 48 and 50 connected respectively to conductors 52 and54 which are connected at opposite center junctions of the rectifier.Conductors 56 and 58 lead from the reversing switches'to the chuck 24.

The AC interrupting switch 44 and the two reversing switches 48 and 50are operated in unison, as indicated in FIGS. 3 and 4 and as describedmore-fully in my prior patent mentioned. The three switches aresooperated by switching bars 60 and 62 on opposite sides of the twobanks of switches. The contactor bars of the AC operating switch 44 arespecially shaped so as to move in a sequence different from those of thereversing switches, so that a make-before-break action is accomplished,i.e., the AC is interrupted, then the DC circuit is established, on bothsides of the reversing switches, then the DC is interrupted at the firstcontacts, and thereafter the AC is reestablished. The switching means22, as will be understood, is operated in synchronism with the rotarytap switch 20 to accomplish the result just stated, and with the furtherrefinement that the AC interrupting switch 44 is actuated before theswitch arm 30 moves off each contact terminal 32, and againreestablished the AC circuit before that am moves onto the next contactterminal. The motor 18 operates through a complete predetermined cyclein a demagnetizing operation, after the operation thereof is initiatedby the manual control of the operator, in which the magnetization of thechuck is reduced to zero, or nearly zero.

A relay A has its coil connected in the conductor 58, and adapted to beshunted by a relay switch Bl. Also connected in the conductor 58 is apotentiometer or adjustable resistor 64, adapted to be shunted by relayswitch CD1.

The relay A is of a permanent magnet type, of known kind, wherein uponenergization thereof, as'by a current in the conductor 58, the switchesthereof are thrown to their abnormal position by the energized magnet,and are so retained upon cessation of that current, and are not moved totheir normal position until the magnet is demagnetized. Uponenergization of the magnetic chuck 24, the relay A is thus actuated,closing switch Al and turning on a signal light as (red) to indicatethat fact. Thereafter, the circuit to the chuck is opened, in a mannerdescribwd hereinbelow since it is desired not to retain the circuitclosed thereto, because of the permanent magnet characteristics of thechuck. However, in the present instance, even after the circuit to thechuck is opened, the chuck remains magnetized until it is controllablydemagnetized, and the signal light so also remains on until the relay Ais demagne'dzed, which is done in the same operation in which the chuckis dcmagnetized.

Additional control is exerted on the chuck through a unit indicated inits entirety at 7% and which includes a 292i tube '"2. operation of thisunit 763* in the present instance is similar to that disclosed andclaimed in my prior Pat. No. 3,045,15 i dated lo]. 17, 1962, to whichreference may be had for greater detail thereof. unit ill is utilizedfor effecting or controlling RESIUUAL, or partial demagnetization of thechuck. The tube 72. is preferably maintained constantly in conductingcondition, through an adjustable resistor 74 which reflects theimposition of a positive bias on the tube. The adjustable resistor inthe conductor is arranged for impressing a controlling bias on the tubeto render it nonconducting upon a predetermined condition, such as areverse current in the conductor 58, Specifically, the latter controloperation is produced by a negative current signal produced in theconductor in a demagnetizing operation. Assuming a positive currentindicated by the arrow 7s utilized for energiz ing the chuck (when theswitch CD1 is closed), upon a demagnetizing current in the oppositedirection indicated by the arrow ill being established (with the switchCD11 then open), the desired negative signal is produced and imposed onthe tube 72. This reverse direction current builds up at a certain rate,and upon the magnetization of the chuck decreasing to the desired levelaccording to the setting of the adjustable resistor 64 and consequentdevelopment of a signal by that current, the signal effectsextinguishing of the tube 72. and opening of the circuit is controlledthereby. This opening of the circult is efiected through deenergizationof the l) relay coil resulting in control functions in other portions ofthe circuit.

As an initial step in the operation of the apparatus, it is firstactuated to charge or energize the chuck 2 3 to hold a workpiecethereon. To do so, the VAlRlABLE-FULL switch $4 is set to FULL position,and the CHARGE-RELEASE switch 82 is set to CHARGE position; a circuit isthen completed as follows: conductor 84 from one center junction of therectifier as, the switch 82, conductor as, the switch b0, conductor 88,the upper contacts of a RECllARGE switch 96}, conductor 92, switch E1 ofrelay E, limit switch 31, conductor 94, through the forward field coilof the motor 13, conductors 98 and w ll: to the opposite center junctionof the rectifier 26. The motor is then driven to move the switch Ellfrom an OFF position indicated in H6. 6, to an ON position and then to aRESlDUAL position. in the latter position, the switch 26? is in circuitclosing position, as are all of the components of the switching means232, and as a consequence charging or energizing current is impressed onthe chuck 24.

The circuit to the chuck established by the foregoing step justdescribed is interrupted shortly after its completion and consequentfull charging of the chuck through the limit switch 33, the camcontrolling this switch being so set that it opens the switch after thedesired predetermined interval and at such position of the motor androtary tap switch 2i) that the arm Bil of the latter is in an OFFposition between active contact terminals (F1 8. 43) and with the Allinterrupter switch 44 open. Althougl the switch interrupts the circuitto the chuck, the relay A coil remains energized because of itspermanent magnet nature, as described above.

As indicated above, it is often desired to remove one or more workpiecesfrom the chucir, to check it or to check the operation being performedthereon, or for other reasons. in such case it is desired not tocompletely demagnetize the chuck, as through a complete demagnetizingcycle, but to reduce the magnetization to a desired degree, usuallygreater than zero, and this preferably by a quick operation. This can beeffectively accomplished in the use of the present invention by theRESEDUAL control, consisting chiefly of the unit 7b which operates toimpose a reverse current on the chuck until the magnetization decreasesto the desired level.

To perform this operation, the operator actuates the RESlDUAL pushbutton M32, which includes two switches W211 and llllZb. Upon depressionof the pushbutton W2, and particularly upon closure of switch 102athereof in the unit ill, a circuit is completed through the circuit ofunit 7b as follows: beginning with the residual switch llllZa, throughthe switch ltl l, conductor 3%, switch B2, conductor llld, the tube 72,the adjustable resistor 74, conductors llll and M2, and secondary lids,the relay D coil, conductor llll l, switch A2 in return to the residualswitch lillla. it will be recalled that at this time the switch A2 isclosed due to the energization of the A coil in response to theenergization of the chuck. The circuit of the unit 7% is retainedthrough switch D2;

Energization of the D relay coil, and consequent closure of switch Di,energizes the CU relay coil in the following circuit: beginning withswitch D ll, conductor lid, switch lll2b, conductors lid and El E9,switch 8h, conductor tilt and conductor $4 to one junction of therectifier as; the circuit continues from the opposite junction toconductor i212, conductor 12.4 through the CU coil and in return to theswitch Di. Energization of the CU coil closes the switch CU2; andimposes a current in the reverse direction on the chuck, the arm Bill ofthe rotary tap switch Ell being at this time in the RESIDUAL position,indicated at E la in dotted lines, and the reversing switches db andSill being in such position as to impose a current on the chuck oppositethat at which it was charged; the circuit thus set up to the magneticchuck is as follows; from tapping 22d of the secondary llZSll, theassociated conductor 34a, conductor l2ti, the switch CU2, conductor l3llto the contactor arm 36. thus completing the circuit to the rectifier doas described above, and of course completion of the DC circuit to thechuck. This reverse current as controlled through the adjustableresistor or produces the signal, as described above, which extinguishesthe tube 72 and opens the circuit of the unit 79, deenergizating the Drelay coil, as a result of which the CU relay coil is deenergized,opening the switch CUZ and interrupting the circuit to the chuck. Thereverse current flowing through the A relay coil also demagnetizes thiscoil and deenergizes this relay, opening switch All and turning off thesignal light 68. Deenergization of the A relay coil closes the switch A3and turns on a signal light 132 (white). The circuit to this light is:beginning with switch A3, conductors i2 3 and 122 to one junction of therectifier 2b; the circuit continues through the opposite junction of therectifier through conductor 84, through the switch 32, conductor 129,switch 8t conductors 11% and 134, through the light i332, through thenormally closed switch CUZl, and in return to the switch A3.

in this RESHDUAL operation, it is desired that there be at least somemagnetization in the chuck in order to retain the workpieces in positionthereon, and not risk accidental dis placement of them, since in may ifnot in most instances, the working operation is to be continued, and itis desired that the workpieces that are not removed remain in theiroriginal positions. The control unit 74) thus serves as a means forquickly demagnetizing the chuck and workpieces to the desired degree ofmagnetization without, however, in all cases, and as desired, reducingthat magnetization to zero, and without the necessity for a full cyclingoperation of the demagnetizing portion of the apparatus which includesthe rotary tap switch Ztl, switching means 22, etc.

After the above RESlDUAl, operation is performed, and Wl'lfii'l it isdesired to return the apparatus again to full power condition, RECHARGEswitch t ll is depressed, closing the contacts at the bottom thereof;this completes the circuit as follows: beginning with the switch itself,conductor the switch 86, conductor 86, switch 82, conductor 84 to onejunction of the rectifier 26; in the other direction from the switch @bthe circuit continues through conductor lilo, relay E coil, resistor13%, conductors Mil, E42, M4 and M6 to the reverse field coil M8 of themotor 118', the circuit then continues through conductors 9% and 1100 tothe opposite junction of the rectifier 26.

Upon energization of the E coil in the circuit just referred to, theswitch E2 is closed, and a retaining circuit is set up as follows:beginning with the switch E2, conductors 150 and $4 to one junction ofthe rectifier 26; and in the opposite direction through the E coil andthe remainder of the circuit as just described, namely, resistor llllil,conductors 14%, M2, 144 and Mt: and M6 to the reverse field 148 of themotor and continuing through conductors 98 and Hill) to the rectifier26. Also, when the E coil is energized, it completes a circuit throughthe holding switch E2, as follows: beginning from the switch E2, thecircuit in one direction is through conductors llfill and lid to theswitch 82, conductor as to the switch 80, conductor 88, the uppercontacts of the switch 90, conductors 92 and 4, abnormally closed switchE3, conductors Mil, 142, i144 and 1 th to the reverse field coil led ofthe motor, and continuing through conductors 9E and lltlll to theopposite junction of the rectifier 26. This circuit operates the motor18 to the fullon power position, and when that position is reached, thechuck is again charged as previously described, the circuit then beinginterrupted by movement of the limit switch 29 to an OFF positiondeenergizing the E relay coil, this control of the motor being throughthe corresponding cam 35. Thereafter, the motor is then reversed, andrun to the residual position through the following circuit: conductor 84from one junction of the rectifier, the switch 82, conductor 86, theswitch 85), conductor 8d, the upper contacts of the switch 90, conductor92, normally closed switch El and the limit switch 31; the circuitcontinues through conductors 150, 94 and 152 to the forward field coil96, and then through conductors 98 and Hill to the opposite junction ofthe rectifier 26. This runs the motor until it reaches the RESHDUALposition in which the limit switch 3i reaches an intermediate offposition.

When the operation on the workpiece is completed, and it is desired toremove it from the chuck, the chuck and workpiece are demagnetizedcompletely, or to a condition near zero magnetization. To demagnetize,the switch 82 is thrown to RELEASE position shown in dotted lines. Acircuit is then completed, as follows: from one junction of therectifier 26 through conductor 84, the switch 82 conductor 156 to thelimit switch 27, and continuing through conductors lSh, 94, and 154 tothe forward field coil as of the motor and then through conductors 98and MM) to the opposite junction of the rectifier 26. The motor is thenrun in forward direction, driving the rotary tap switch through a fulldemagnetizing cycle as set out in detail in my prior Pat. No. 2,825,854,referred to above, the cycle terminating when the limit switch 27 isopened, under the control of the corresponding cam 35, the switch comingto rest in an intermediate position, deenergizing the motor. in thisdemagnetizing operation, the motor stops at such position that the arm30 of the rotary tap switch 20 is in an OFF position (H6. 6) betweencontact terminals 332, and the AC interrupter switch 44 is open.

The apparatus also provides for a VARIABLE holding power condition, inwhich the magnetization of the chuck is substantial, but less than fullpower of which the apparatus is capable of producing. This may be usefulin an instance in which, for example, it is desired to hold a workpieceon the chuck with less than full power so as not to distort the piece.instances occur in which workpieces may be distorted by the holdingforce of the chuck, provided by the full capacity power of theapparatus. This variable, or lesser, holding power is controlled throughthe use of a variable auto transformer Mil, which is set to the desiredposition manually, for producing a holding power less than maximum.After setting this auto transformer, the (IliARGE-RELEASE switch 82 ismoved to CHARGE position, and the VARIABLE-FULL switch fill moved toVARIABLE position shown in dotted lines. These settings produce thefollowing circuit: from one junction of the rectifier 26 throughconductor 84, the switch 82, conductor 86, through the switch 80,conductor 162, relay E coil, conductor 16d, and through limit switch 33which is then in the position opposite that shown; the circuit thencontinues through conductors HM and 154 to the forward field coil of themotor 18, and then through conductors 98 and lilll to the rectifier. Therelay B coil energized by this circuit closes a holding switch B3; acircuit is also completed from conductor 84 through conductor 166, andthrough the switch to conductor ids, abnormally closed switch B4,conductor H70, through conductor 88, and the upper contacts of theswitch 9%, conductor 92, switch El and through the limit switch 3i; thecircuit then continues through conductors 152, 1% and B54 to the forwardfield coil 96 of the motor and through conductors 98 and 100 to therectifier. The motor is then run in forward direction to a FULL powerposition in the manner described above, but producing a value ofmagnetization as determined by the setting of the variable autotransformer lath. This auto transformer is connected in the AC circuitthrough the rotary tap switch 20, as follows: from the auto transformer,conductor H72, abnormally closed switch 86, conductor 1174, rotary tapswitch 29, conductor 38, AC interrupter switch 44, and conductor is toone junction of the rectifier 40; the circuit continues in the otherdirection from the auto transformer through conductor 1176, abnormallyclosed switch B7, conductor Nd, conductor lliil, the transformersecondary i251 and conductor 36 to the opposite junction of therectifier 49.

The motor continues to run in this VARlABLE setting operation until thecircuit is opened by the corresponding limit switch 27, which iscontrolled by the corresponding cam 35 in the same full power sequenceas described above, with the exception that the power applied to thechuck is at a lesser value, and proportionate to the setting of thevariable auto transformer 16).

if more power is required, i.e., greater value of magnetization of thechuck, the variable auto transformer 1160 is set at the appropriatehigher setting and the switch @d is again actuated; the apparatus willthen operate through the sequence just described. If after a first orany subsequent VALE setting it is desired to have full power applied,the switch $0 is thrown to FULL position, and the switch 9il actuated;the chuck will then be charged at full power, as in the case when it isto be charged from a zero magnetization condition. When it is desired todemagnetize the chuck from a VARIABLE condition, the switch 82 is thrownto the RELEASE position, as in the case of FULL power condition, and thedemagnetizing cycle is performed.

Attention is now directed to FIG. 2, showing a circuit diagram of amodified form of the invention. The present circuit includes certain ofthe same components incorporated in the circuit of FIG. I, andidentified herein in the same manner, as follows: the AC source ill,transformer 12, motor 18, rotary tap switch 20, switching means 22,rectifier 49 and chuck means 24 which in the present instance includes aplurality of, or two, chucks 24a and 24b. The conductors leading fromthe switch means 22 to the chuck means include the conductors 56 and 53identified in the first embodiment, but here having branch portions 56aand 56b and 58a and 5817, respectively, leading to the individualchucks. Associated with the motor B8 are the limit switches 27, 29, Eliand 33 identified above. Other portions of the present circuitcorrespond to similar portions of the circuit in the first embodiment,as will be observed.

The present form of the control apparatus includes RESlDUAL units 1% and192, including 2521 tubes li land E96 respectively, These two units aresimilar operation to the unit 763 in that they interrupt the circuit tothe chuck means in response to decrease of the magnetization in thechuck means in a partial demagnetizing or RESlDUAL demagnetization,under controlled conditions, according to a predetermined manualsetting.

The RESHDUAL control unit 1190, considered the base unit, includes arelay F coil, and a holding switch Fl, there being an associated switchF2 in the unit W2. The circuit of this unit also includes a switch Glcontrolled by a G relay coil in the RECHARGE circuit portion andcorresponding to the relay E in the first modification.

in the DC conductor 5% leading to the chuck means is a variable resistor3953 similar in function to the corresponding component 64 in the firstmodification, for imposing a controlling bias on the tubes 1% and 11%.Upon reverse current flowing in the conductor 5%, a negative signal isproduced and imposed on the tube (1%) to open the circuit of that unit(il ll). Upon the F coil becoming thus deenergized, the switch F2 opens,the KU coil is deencrgized with consequent opening of switch kill, andopening of the circuit to the chuck. This unit may be set by adjustingthe adjustable resistor 11% to the desired setting for interrupting thecircuit to the chuck means at a predetermined level of magnetizationthereof when it is desired, for example, to partially demagnetize theworkpieces to remove one or more of them for test purposes, as explainedabove.

The unit 192 may be referred to as an auxiliary unit which operateswithin the base range established by the unit 1%. This unit includes arelay coil and a holding switch ill, the li coil controlling acorresponding switch H2. The unit 192 has particular adaptation to ammhine tool in conjunction with an operation of the following character:it often occurs that a workpiece is distorted when placed on a magneticchuck and held thereon by a large magnetic force, due to any of variousreasons, such, for example, as an irregular shape of the piece. Whensuch a piece is held on the chuck by a large magnetization force, and sodistorted, and a working operation performed thereon, such as a grindingoperation, the desired surface, such as a flat surface, is apparentlyproduced on the piece, but this is only when considered relative to itscondition under full magnetization holding effect and when the piece isreleased from the chuck by removal of the magnetization force, itassumes, or tends to assume, its original shape, and in doing so, thesurface that was regarded as a flat surface when the workpieces was heldon the chuck changes to an irregular or nonflat surface. This difficultyis not easily overcome, because such large value of magnetization as maydistort the piece is necessary to hold the piece on the chucksufficiently securely for the working operation, and if it were heldwith less force, it may be moved by the operating instrumentality, suchas the grinding wheel. I have overcome this difficulty by providing theunit 192, by means of which the workpiece can be held in place insuccessive steps by progressively diminished magnetization force, andthe working operation perfonned thereon in successive steps may involveless operating power.

The signal developed in the adjustable resistor 198 is also imposed onthe tube 1%, through the conductors 2G0, 202 and 204, but this signal iscontrolled by a manually settable potentiometer 206. The control we,acting through the circuit of the unit 192, including the secondary1234, controls the effectiveness of the sigial produced by theadjustable resistor, so that the unit 192 interrupts the circuitaccording to the setting of the potentiometer sun, which is at alevel'of magnetization less than that which controls the unit 190. Thesignal causes opening of the circuit of the unit, thereby deenergizingthe H coil, opening the switch H2, deenergizing the lfiU coil, andopening the switch Kill with consequent opening of the circuit to thechuck. The effectiveness of the unit r92 may be realized from anoperation such as the following: a workpiece that may be distorted isplaced on the chuck and held thereon by full magnetization force; it isthen ground, the circuit is interrupted according to the RESIDUALoperating, which results in lesser magnetization imposed on theworkpiece, and it tends to assume its original shape. A second operationis then performed thereon, and after that is done, the circuit is againput through the RESTDUAL cycling, and the chuck magnetized at a stilllesser level. Another working operation is performed on the workpiece.At each step the workpiece assumes a condition closer to its originalundistorted condition, and the working operation thereon is lighter,e.g., the grinding or cutting operation is shallower. in this series ofoperations, it is desired not to remove the workpiece from the chuck, soas to eliminate the possibility of foreign matter coming between theworkpiece and the chuck, which would disturb its original positionthereon.

Fit]. 7 indicates such a condition wherein a workpiece 2% is held on achuck. This H6. is greatly exaggerated for more clearly bringing out thefeature involved. in position A the chuck is deenergized, and theworkpiece 2698 thereon is in its normal condition, having an upperconvex surface Zltla and an under concave surface Zlllb.

in position B the chuck and the workpiece are magnetized, and themagnetization force draws the workpiece downwardly toward a flatposition. it will be understood that the actual change in position in apractical operation may be so small as to be undetectable by the nakedeye, but as indicated above, the conditions and p roportions indicatedin these FIGS. are many times multiplied relative to a practicaloperation. A grinding wheel 212 is set to grind a surface on theworkpiece represented by the dot-dash line 214, which is flat, in thecondition of the workpiece when it is held in its relatively flattenedposition.

When the chuck and workpiece are thereafter demagnetized, the workpiecetends to reassume its original condition, as indicated in position C,and here it will be observed that the outer surface 210a is againarcuate. However, the surface of the workpiece which was once flat inposition B where the workpiece was relatively flattened, is now againcurved. Thus, a flat surface in this step is not attained non theworkpiece in its normal condition because of the distortion of theworkpiece in the holding operation, and the resumption of its originalshape when the magnetization is removed.

The central portion of the workpiece after the grinding operation, asindicated at 212a (position C), is thinner than the end portionsindicated at 2l2b. This condition is obviated in a succession ofgrinding operations of the kind just described, but with the workpieceheld by progressively lesser forces, i.e., each time the chuck andworkpiece are magnetized, they are so magnetized to a lesser degree sothat the distortion of the workpiece is progressively less in thesuccessive steps. in this same series of operations, successivelyshallower cuts are taken, and because of the less force thus applied, itis unnecessary to utilize so great magnetization power to hold theworkpiece. Such series of progressively lesser demagnetization andshallower cuts are continued until, in the final step, the distortion isat a minimum, and from a practical standpoint substantially eliminated,whereby the surface that is flat when the final light magnetization isapplied, remains substantially flat after the magnetization force isremoved.

The same kind of operation can be perfonned on the opposite side of theworkpiece, as indicated in positions D and E of Fit]. 7. In position Dthe same workpiece is placed on the chuck with the convex side down;'inthis case also, when the chuck is magnetized, as indicated in positionE, the workpiece is flattened. When the grinding operation is performedfor producing a flat surface at the line 214, the same kind of result isobtained as described above. In a series of progressively decreasedmagnetization, and shallower cuts, the concave side of the workpiece canbe provided with a flat surface in the same manner as on the convexside, described above. Position F illustrates workpiece 208 in which theconvex and concave portions have been ground in the manner abovedescribed, these latter portions being indicated by the dotted lines,resulting in a workpiece as indicated by the dotted lines, resulting ina workpiece as indicated in full lines that has fiat surfaces on bothsides.

Although in FIG. 2 there are two chucks illustrated, it will beunderstood that the apparatus is effective in connection with any othernumber of chucks, including a single chuck, or a greater number thereof.The invention is also effective for individually controlling a pluralityof chucks. For example,

switches 11 and .11 are provided in the conductors 56a and 56b,respectively, controlled by respective relaycoils I and J, which in turnare controlled by manually controlled switches 218a and 218b, related tothe respective chucks 24a and 24b. A similar arrangement may be providedfor accommodating any desired number of chucks.

While I have herein shown and described certain preferred embodiments ofthe invention, it will be understood that changes may be made in thescope of the appended claims.

lclaim:

1. The method of treating a workpiece which is of rigid structure andnormally retains its physical shape in the absence of the application ofexternal pressure thereto, and of which the final shape is to becritical, comprising the steps,

a. holding the workpiece in place by a force of predetermined firstvalue, and of such value that it incidentally but necessarily distortsthe workpiece,

b. performing a first operating step on the workpiece involving theapplication of pressure thereto of a first value approaching that thatwould dislodge the workpiece if it were not held by a holding force atleast as great as said predetermined first value thereof,

c. after the performance of the first operating step, reducing saidholding force to a second value less than its said first value thereofbut greater than'zero, that also incidentally but necessarily distortsthe workpiece but to an extent less than the distortion caused by theholding force of said first value, and I d. performing a successiveoperating step on the workpiece involving the application of pressurethereto of a value less than said first value thereof and of a valueapproaching that that would dislodge the workpiece if it were not heldby a holding force at least as great as said second value thereof.

2. The method set out in claim 1 wherein the treating operation includesremoval of material from the workpiece, and the amount of materialremoved is reduced in successive steps.

3. The method according to claim 1 and including the additional step ofreducing the holding force to a level near zero and thereby releasingthe workpiece.

4. The method according to claim 3 wherein the holding force is providedby a magnetic chuck, and the method includes reducing the magnetismthereof in the successive steps.

1. The method of treating a workpiece which is of rigid structure andnormally retains its physical shape in the absence of the application ofexternal pressure thereto, and of which the final shape is to becritical, comprising the steps, a. holding the workpiece in place by aforce of predetermined first value, and of such value that itincidentally but necessarily distorts the workpiece, b. performing afirst operating step on the workpiece involving the application ofpressure thereto of a first value approaching that that would dislodgethe workpiece if it were not held by a holding force at least as greatas said predetermined first value thereof, c. after the performance ofthe first operating step, reducing said holding force to a second valueless than its said first value thereof but greater than zero, that alsoincidentally but necessarily distorts the workpiece but to an extentless than the distortion caused by the holding force of said firstvalue, and d. performing a successive operating step on the workpieceinvolving the application of pressure thereto of a value less than saidfirst value thereof and of a value approaching that that would dislodgethe workpiece if it were not held by a holding force at least as greatas said second value thereof.
 2. The method set out in claim 1 whereinthe treating operation includes removal of material from the workpiece,and the amount of material removed is reduced in successive steps. 3.The method according to claim 1 and including the additional step ofreducing the holding force to a level near zero and thereby releasingthe workpiece.
 4. The method according to claim 3 wherein the holdingforce is provided by a magnetic chuck, and the method includes reducingthe magnetism thereof in the successive steps.